Can't fix "Error loading operating system" message

[gabrielantonio]

HI,

I have (well, had) a working Win7 installation, which I installed from a USB hard drive. I want to dual boot XP, so I started to install it and, when it rebooted, got the "Error loading operating system" message, which was expected since XP wipes out other MBRs. No biggie, I can just boot to a prompt using the Win7 disc and fix it there, right?Nope.
When I try that I get "Press any key to boot from CD" and then a blank screen with nothing but a flashing cursor. ::sigh:: Okay, so let's boot using the USB drive I installed Win7 from.Nope.
When I do that I get "Error loading operating system" again. WTF?

Any help will be apprecited.

I didn't find the right solution from the Internet.

References:
https://arstechnica.com/civis/viewtopic.php?p=22100830
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Thank you.

[Velbloud]

I don't know, sorry.

[Elfus]